1. Field of the Invention
The present invention relates to a hydrostatic transmission (hereinafter referred to as “HST”) in which a hydraulic pump and a hydraulic motor are fluidly connected to each other via a closed fluid circuit, the HST including a fluid charging means for supplying fluid to the closed fluid circuit.
2. Related Art
A typical HST includes a hydraulic pump, a hydraulic motor, and a closed fluid circuit through which the hydraulic pump and motor are fluidly connected to each other. The closed fluid circuit includes a pair of main fluid passages interposed between the hydraulic pump and motor. The hydraulic pump is driven by an engine or the like so as to supply fluid to the hydraulic motor through the closed fluid circuit, thereby driving the hydraulic motor. The closed fluid circuit has to be supplied with additional fluid because it inevitably leaks fluid during operation. The lack of fluid in the closed fluid circuit can cause air bubbles in the fluid which causes inefficient reaction of the hydraulic motor.
Therefore, as disclosed in JP 2007-203972 A, the typical conventional HST is usually provided with an additional charge pump for supplying fluid to the closed fluid circuit. To drive the charge pump, a pump shaft of the hydraulic pump is used as a drive shaft of the charge pump, or an additional electric motor is provided. A fluid charging passage is provided for receiving fluid from the charge pump to be supplied to the closed fluid circuit. If the main fluid passages of the closed fluid circuit are formed in a center section on which the hydraulic pump and motor are mounted, the fluid charging passage is normally formed in the center section.
However, the addition of the charge pump increases the number of parts and costs. Further, if the charge pump is disposed apart from the closed fluid circuit, pipes have to be provided to fluidly connect the charge pump to the closed fluid circuit, and the pipes have to be prevented from interfering with other surrounding implements. If the charge pump is assembled in the HST, the HST is inhibited from being minimized.